Hydraulic jars, most commonly used as drilling jars, are a common category of drilling jar that has been in use for many decades. A prominent feature of this category of drilling jar is that when a tensile load is applied the jar will telescope open slowly for the initial phase often referred to as hydraulic delay. The hydraulic delay is created by a fluid, typically hydraulic oil, passing through a region of high resistance such as a small orifice from a high pressure chamber to a low pressure chamber. An example of a prior art drilling jar is shown in FIG. 1.
As the tensile force increases, the pressure in the high pressure chamber also increases. A limitation of this category of jar occurs when the pressure in this chamber becomes excessive. Excessive pressure may cause the outer housing to rupture, the inner mandrel to collapse, the seals to fail, or a combination of these. This limitation is more prominent with drilling jars that have reduced wall thickness. Larger inner diameter drilling jars typically result in increased pressure for the same tensile force.
A second category of drilling jar has the hydraulic delay feature mentioned above in addition to a pressure relief valve. This feature prevents the jar from telescoping open until the applied tensile force is high enough to cause the pressure relieve valve to open. A limitation of this design is the maximum pressure these pressure relief valves release at.